The Pacific lamprey is distributed throughout the Pacific Rim from Japan to Mexico. Lampreys belong to a primitive group of fishes that are eel-like in form but lack the jaws and paired fins of true fishes. It is from this resemblance that they are sometimes called eels. Lamprey have a round sucker-like mouth, no scales, and breathing holes instead of gills.
As adults in the marine environment, Pacific lamprey are parasitic and feed on a variety of prey. After spending one to three years in the marine environment, they cease feeding and migrate to freshwater in the spring. They are thought to overwinter and remain in freshwater habitat for at least a year before spawning the following summer. As larvae, lamprey spend approximately four to six years burrowed in fine silt and sand filter feeding before metamorphosing into juveniles and migrating to the ocean.
Due to their unique life history, the Pacific lamprey is an important component to the ecosystem both as a predator and prey. As a predator, Pacific lamprey has coevolved with native fish assemblages in the Pacific Ocean and freshwater ecosystems. As prey, lamprey is important to many species of fish, birds, and mammals, including humans. Like other anadromous fish species, they transport nutrients from the ocean to the freshwater environment.